1. Technical Field
The present invention relates to flow measuring devices, in particular, to a flow measuring device for measuring a flow amount of a gaseous body.
2. Related Art
A straight-tube type flow measuring device typically measures a flow velocity V of a gaseous body that flows through a flow channel 12 using a flow detection element 11, as shown in FIG. 1A or 1B, and measures a flow amount of the gaseous body within the flow channel 12 based on the measured flow velocity V and a flow channel cross-sectional area.
When a certain amount of the gaseous body flows through the flow channel 12 in the flow measuring device having such a structure, the flow velocity V of the gaseous body increases if the flow channel cross-sectional area is small as shown in FIG. 1A, and the flow velocity V of the gaseous body decreases if the flow channel cross-sectional area is large as shown in FIG. 1B. On the other hand, the flow detection element 11 has a predetermined measurable range, and if the flow velocity V of the gaseous body falls outside the measurable range, measurement accuracy for the flow velocity V decreases or the flow velocity V becomes unmeasurable. Therefore, for a straight-tube type flow measuring device that directly measures a large flow amount of the gaseous body, for example, 200 liters/min (hereinafter abbreviating liters/min to L/min), it is necessary to increase a diameter of the flow channel 12 in order to reduce the flow velocity V so as to fall within the measurable range of the flow detection element 11, which restricts downsizing of the device.
Therefore, in order to downsize a flow measuring device for measuring a large flow amount, a flow measuring device of a bypass flow channel structure as shown in FIG. 2 has been proposed. In the flow measuring device of the bypass flow channel structure, an auxiliary flow channel 14 is branched from a main flow channel 13, and an orifice 25 is provided within the main flow channel 13. Due to a differential pressure produced in the vicinity of the orifice 25, a part of the gaseous body that flows through the main flow channel 13 is directed to the auxiliary flow channel 14, and a flow velocity Vp of the gaseous body that flows through the auxiliary flow channel 14 is measured by the flow detection element 11. Then, a total flow amount is derived based on such as a flow ratio between the gaseous bodies that flow through the main flow channel 13 and the auxiliary flow channel 14, the flow velocity Vp in the auxiliary flow channel 14, and a flow channel cross-sectional area of the auxiliary flow channel 14.
Japanese Patent No. 3870969
As an example of the flow measuring device of the bypass flow channel structure, a device disclosed in Japanese Patent No. 3870969 is known. As shown in FIG. 3, in a flow measuring device 21 of this example, the main flow channel 13 is formed within a main flow tube 22 and the auxiliary flow channel 14 is formed outside the main flow tube 22, an inlet 23 and an outlet 24 that open inside the main flow channel 13 respectively communicate to both ends of the auxiliary flow channel 14. Further, the orifice 25 is provided within the main flow channel 13 between the inlet 23 and the outlet 24, and a flow ratio between the main flow channel 13 and the auxiliary flow channel 14 is adjusted according to an opening size and a shape of the orifice 25.
However, according to the flow measuring device 21, in order to reduce a number of component parts and a production tolerance of the flow ratio, the orifice 25 is integrally formed with the main flow tube 22. Accordingly, it is not possible to change the opening size and the shape of the orifice 25, the main flow tube 22 as a whole has to be redesigned and remanufactured when trying to change the opening size and such of the orifice 25. As a result, preparing flow measuring devices of flow amount ranges for various purposes becomes expensive and requires a large storage space.
On the other hand, it is possible to form the orifice 25 separately from the main flow tube 22, as shown in FIG. 4, in order to make the orifice 25 replaceable to commoditize the main flow tube 22. In this case, as shown in FIG. 4, it is possible to make the orifice 25 to be contained within the main flow channel 13, and makes a whole circumference of the orifice 25 adhered to a wall surface of the main flow channel 13 with an adhesive agent 26.
However, according to this method, a number of the steps of assembling the flow measuring device increases due to a step of applying the adhesive agent 26 and a thermal curing process of the adhesive agent. Further, the method of making the orifice 25 applied with the adhesive agent 26 to be contained within the main flow channel 13 may decrease the measurement accuracy because the adhesive agent 26 adheres to the wall surface of the main flow channel 13 (in particular, in the vicinity of the inlet 23). Furthermore, even with the method of applying the adhesive agent 26 to the wall surface of the main flow channel 13 in advance and then making the orifice 25 to be contained afterwards, the adhesive agent 26 can protrude from a portion between the orifice 25 and the wall surface of the main flow channel 13, thereby causing the decrease of the measurement accuracy, and requiring the management of the adhesive agent.
Japanese Unexamined Patent Publication No. 2004-355045
The device disclosed in Japanese Unexamined Patent Publication No. 2004-355045 is one example of the flow measuring device of the bypass flow channel structure in which the orifice is formed separately from the main flow tube and the orifice is fixed within the main flow channel without using the adhesive agent. According to a flow measuring device 31 disclosed in Japanese Unexamined Patent Publication No. 2004-355045, as shown in FIG. 5, the orifice 25 and a collar 32 having a cylindrical shape are contained in the main flow channel 13, and the orifice 25 is sandwiched and held between a stopper section 34 formed within the main flow channel 13 and the collar 32. An end surface of the collar 32 and the stopper section 34 are respectively provided with fitting holes 35 and 37, and a supporting pin 38 press-fitted into the fitting hole 35 of the collar 32 is inserted through a through hole 36 of the orifice 25 and through the fitting hole 37 of the stopper section 34. With this, the collar 32 is baffled and a vent hole 33 opening in the collar 32 is positioned so as to be aligned with the inlet 23. Moreover, gas flow straighteners 39 and auxiliary spacers 40 are alternately contained at a rear of the collar 32.
Further, according to the structure as described in Japanese Unexamined Patent Publication No. 2004-355045, it is required to insert the supporting pin 38 into the fitting hole 37 of the stopper section 34 deep within the main flow channel 13, and the insertion is not easily carried out. Accordingly, as shown in FIG. 6B, a method of baffling the collar 32 by providing a projection 42 protruding around an outer circumference of a back end portion of the collar 32, and fitting the projection 42 into a depression 41 provided for the wall surface of the main flow channel 13 as shown in FIG. 6A can also be conceived.